A Digital Calibration Source for 21cm Cosmology Telescopes

TL;DR

This paper introduces a digital calibration source for 21cm cosmology telescopes that uses GPS-timed signals for precise phase and amplitude calibration, improving foreground mitigation techniques.

Contribution

It presents a novel GPS-based digital radio source for interferometric calibration, with initial implementation and beam mapping results, advancing hardware calibration methods.

Findings

The digital source can be correlated with local copies for phase and amplitude calibration.

Commercial SDR boards partially meet the requirements, indicating need for more advanced hardware.

Beam mapping demonstrated the potential of the digital source for antenna calibration.

Abstract

Foreground mitigation is critical to all next-generation radio interferometers that target cosmology using the redshifted neutral hydrogen 21 cm emission line. Attempts to remove this foreground emission have led to new analysis techniques as well as new developments in hardware specifically dedicated to instrument beam and gain calibration, including stabilized signal injection into the interferometric array and drone-based platforms for beam mapping. The radio calibration sources currently used in the literature are broad-band incoherent sources that can only be detected as excess power and with no direct sensitivity to phase information. In this paper, we describe a digital radio source which uses Global Positioning Satellite (GPS) derived time stamps to form a deterministic signal that can be broadcast from an aerial platform. A copy of this source can be deployed locally at the instrument correlator such that the received signal from the aerial platform can be correlated with the local copy, and the resulting correlation can be measured in both amplitude and phase for each interferometric element. We define the requirements for such a source, describe an initial implementation and verification of this source using commercial Software Defined Radio boards, and present beam map slices from antenna range measurements using the commercial boards. We found that the commercial board did not meet all requirements, so we also suggest future directions using a more sophisticated chipset.